A heart can be considered the center of a circulatory system within a body. For example, the heart can take deoxygenated blood from elsewhere in the body and provide it to the lungs to be oxygenated. The heart can then supply the oxygenated blood from the lungs to other parts of the body. In a healthy heart, each chamber generally contracts in a coordinated fashion, such as to provide adequate circulation of oxygenated blood and nutrients to sustain the body.
The heart can be affected by a variety of physical and electrical abnormalities. Physical abnormalities can include, among other things, enlarging of the heart, sometimes associated with ischemia. Electrical abnormalities can include, among other things, various arrhythmias, such as due to infarcts, congenital defects, aging, or one or more other factors. Certain arrhythmias can be life threatening, such as including a ventricular tachyarrhythmia or ventricular fibrillation. In some patients, such life threatening arrhythmias can be detected and terminated using low-energy electrical impulses, such as provided by an implantable pulse generator. Such low-energy electrical impulses can include anti-tachyarrhythmia pacing (ATP), but such pacing is not always indicated or effective for termination of a particular arrhythmia. In cases where ATP is not indicated, or is ineffective, a defibrillation countershock can be provided, such as by an automatic implantable cardioverter defibrillator (AICD). An AICD can provide the defibrillation countershock subcutaneously, epicardially, or using one or more intravascularly-deliverable implantable leads such as located within or near the heart or vasculature.
A fault in the AICD, or in an attached lead or electrode assembly can prevent delivery of cardioversion or defibrillation countershock energy. In some situations, such a fault can present a relatively low electrical impedance to an output circuit included in the AICD. The output circuit can be stressed or permanently damaged while attempting to provide the defibrillation countershock energy into the low impedance.
United States Patent Publication No. 2004/0024424 (Propp et al.) discloses an apparatus and method of measuring a lead impedance of a high energy shock lead before delivery of a high energy therapy to treat a heart arrhythmia, including aborting a prospective delivery of the high energy therapy when a shorted lead is detected.
U.S. Pat. No. 5,224,475 (Berg et al.) discloses an implantable defibrillator provided with a plurality of defibrillation electrodes, which may be reconfigured to define a plurality of defibrillation “pathways.” Berg discloses use of impedance measurements during delivery of high voltage cardioversion or defibrillation pulses to detect overall changes in the performance of the defibrillation pathway between the electrodes, for the purpose of optimization of current density when multiple defibrillation pathways are used simultaneously.
U.S. Pat. No. 5,453,698 (Williams et al.) discloses a method and system for testing an implantable defibrillator output stage and high voltage lead integrity, using a 10-20 Volt pulse discharged from a capacitor through the output stage and leads. Williams discloses measuring a residual voltage remaining on the capacitor after the discharge for comparison with a prior residual voltage measurement.